TRIP steels are a class of advanced high-strength steels that have been gaining popularity in automotive applications due to their ductility and strength. Because TRIP steels are more ductile, they are easier to form than other steels with similar initial yield strengths. Yet TRIP steels have a much higher final part strength which makes them desirable in the production of automobile parts. TRIP steels are typically made up of three microconstituents: polygonal ferrite, bainite, and retained austenite. The retained austenite is present in the form of dispersed particles. The high strength of TRIP steels is due primarily to the presence of a substantial amount of the harder martensite and bainite microstructure phases dispersed in a relatively softer matrix of ferrite. The enhanced formability of TRIP steels (the ability to form parts of complex geometry) is due to the progressive transformation of the steel's retained austenite to the stronger martensite when plastic deformation is induced, such as during stamping. This phenomenon is known as transformation induced plasticity, or commonly referred to by the acronym “TRIP.” Because of this enhanced formability, TRIP steels can be used to produce automobile parts having a more complex geometry than parts produced with other high-strength steels. This allows automobile manufacturers to exhibit more freedom in the design of automobile parts to optimize weight and structural performance. TRIP steels also exhibit greater strength at higher strain levels, making them ideal for crash energy management. Thus, TRIP steels are preferred where structural parts of medium to high strength and complex geometry requiring high formability in stamping are desired. TRIP steels are also ideal for automotive components requiring superior crash performance.
To achieve the combination of strength and formability, TRIP steels require a high alloy content. A typical TRIP steel composition generally includes (by wt. %) carbon 0.10-0.50; manganese 1.00-4.00, chromium 0.00-1.00; molybdenum 0.00-0.50; aluminum 1.00-5.00; titanium 0.00-0.20; niobium 0.00-0.20; and vanadium 0.00-0.20. The remainder of the composition is iron plus any unavoidable residuals present during the steelmaking process. Unfortunately, the compositions required to achieve the desired characteristics of TRIP steels also pose challenges in terms of continuous casting. Because of these compositions, continuous cast sequences of TRIP steels have historically been limited because of the necessity to terminate casting after a sequence of TRIP steel was produced. TRIP steels contain higher levels of aluminum, which tend to combine with certain components of the mold flux used in casting. The resulting combination causes a thick film to accumulate on the caster mold walls, deterring the flow of the molten steel. Hydrogen and nitrogen contained in the molten steel exacerbate the accumulation of the film. However, the present invention involves degassing the steel prior to casting to reduce the levels of hydrogen and nitrogen, resulting in a more flowable steel. This allows for longer cast sequences during TRIP steel production and also uninterrupted casting following transition to other steel grades.